(Meth)acrylic artificial marble which is produced by compounding inorganic fillers such as aluminum hydroxide into a (meth)acrylic resin has various prominent functions and properties such as excellent appearance, soft feeling and the like, and is widely used for counters such as a kitchen counter and the like, washing and dressing stands, waterproof pans, bathtubs, and other architectural uses. Such marble is usually produced by a casting method in which a so-called premix prepared by dispersing inorganic fillers in (meth)acrylic syrup is filled in a mold, and the premix is cured and polymerized at relatively low temperature.
For example, Japanese Patent Application Laid-Open (JP-A) No. 61-26,605 discloses artificial marble in the form of a flat plate obtained by curing a composition comprising (meth)acrylic syrup containing cyclohexyl methacrylate; aluminum hydroxide; and a polymerization initiator having a 10 hours half-life temperature of less than 75.degree. C., under low pressure for long period of time. Further, Japanese Patent Application Laid-Open (JP-A) No. Hei 2-305,842 discloses artificial marble in the form of a flat plate obtained by pouring into a mold a composition comprising (meth)acrylic syrup containing tricyclo[5.2.1.0.sup.2,6 ]decanyl (meth)acrylate; a quartz powder; and a polymerization initiator having a 10 hours half-life temperature of less than 75.degree. C., and curing the composition according to a casting method
However, the compositions disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 61-26,605 and Hei 2-305,842 require as long as 2 or more hours for curing and exhibit extremely poor productivity, since a polymerization initiator having a 10 hours half-life temperature of less than 75.degree. C. is used in each case as a curing agent. Then, if these compositions are heated at a temperature of 100.degree. C. or more for curing for the purpose of reducing curing time so as to increase productivity, inconveniences such as cracking, and whitening occur in the resulting molded articles due to low decomposition temperature of the curing agent.
On the other hand, for improving the problem regarding productivity, there has been effected a method for producing (meth)acrylic artificial marble in short period of time by molding with heating and pressurizing a SMC (sheet molding compound) or BMC (bulk molding compound) which is obtained by thickening resin syrup with a thickening agent.
For example, Japanese Patent Application Laid-Open (JP-A) No. Hei 6-313,019 discloses a method for producing artificial marble in the form of a flat plate by molding, with heating at a temperature as high as 115.degree. C. and pressurizing, a (meth)acrylic SMC or BMC prepared by compounding aluminum hydroxide into (meth)acrylic syrup containing as a main component methyl methacrylate.
However, when (meth)acrylic artificial marble is produced by such a method, there occur inconveniences that vapor of methyl methacrylate is generated in a mold, the surface of the molded article is corroded with the methyl methacrylate vapor, gloss at the corroded part decreases and unevenness in gloss occurs, since molding is conducted at a temperature of not less than the boiling point of methyl methacrylate (100.degree. C.). Such tendency becomes remarkable when the curing temperature is further raised for increasing productivity.
Further, the acrylic artificial marble described in Japanese Patent Application Laid-Open (JP-A) No. Hei 6-313,019 tends to be inferior in hot water resistance, and can not be used for uses such as a bathtub in which particularly high hot water resistance is required. The (meth)acrylic SMC or BMC disclosed in this publication has odor specific to (meth)acrylic monomers, and uncomfortable feeling sometimes may occur in handling. Especially when this SMC or BMC is molded at a temperature as high as 100.degree. C. or more, this tendency becomes remarkable. When artificial marble is produced using the (meth)acrylic SMC or BMC disclosed in this publication, there easily occurs inconvenience that the size of a molded article becomes very smaller than that of a mold and goes out of the dimension tolerance of the molded article. Further, the (meth)acrylic SMC or BMC disclosed in this publication requires aging for a long period of time (24 hours or more) for thickening and exhibits low productivity.
On the other hand, Japanese Patent Application Laid-Open (JP-A) No. Hei 6-219,800 discloses artificial stone in the form of a flat plate for use as an architectural material, comprising 5 to 15% by weight of a (meth)acrylic binder resin using a monomer having high boiling point and 85 to 95% by weight of a natural stone particle. However, the herein disclosed composition looks like a stone since the inorganic filler content is high, and has no marble-like deep gorgeous texture or soft feeling. Moreover, since the inorganic filler content is high, flowability of the material is poor and a molded article having complicated form can not be easily obtained.
Further, the molded articles disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 61-26,605, Hei 2-305,842 and Hei 6-313,019 described above are all flat plates having uniform thickness. For example, when a molded article having varied thickness is produced and when a molded article is produced with an insert screw being buried in the article, inconvenience that dimple and whitening occur on the thickness varied parts and the surface at the insert screw tends to be caused. This tendency is particularly remarkable when the curing temperature is raised for producing a molded article in short period of time.